Ceramic materials are currently viewed as potential substitutes for steel and other metals due to their superior strength at high temperatures, thermal shock resistance, toughness, hardness and chemical oxidation resistance. One particular ceramic, silicon nitride, has been identified for use in high temperature applications such as gas turbines and diesel engines.
Two critical parameters of any silicon nitride ceramic are its high temperature strength and its toughness. Typically, high temperature strength is inversely related to the amount of sintering aid used, while toughness is directly related to the presence of grains having a high aspect ratio, i.e., beta phase grains whose lengths are at least four times their thicknesses.
Processing raw materials into a useful ceramic body commonly requires a number of engineering operations. A typical process entails preparing a silicon nitride powder or a precursor thereof, forming a definitively-shaped compact or "green body" from the powder, and sintering the green body to produce a densified hard, tough ceramic. In the sintering process, the green body is subjected to high temperatures (about 1800.degree.-2000.degree. C.) which facilitate material transport. This transport reduces the pore size and volume between the green body particles and assists in the bonding of adjacent particles, thus producing a strong, dense ceramic. Ceramics used in high temperature applications often require advanced sintering processes, such as gas pressure or pressure-assisted sintering ("GPS") or glass encapsulated hot isostatic pressing ("hipping" or "HIP" or "HIPping"), which subject the green body not only to high temperatures but also to pressures on the order of 30,000 psi.
It has been found that GPS processes are typically conducive to growing grains having a high aspect ratio. During GPS, silicon nitride normally transforms completely from the alpha to the beta phase in the intermediate stages of sintering, during which time beta grain growth and pore elimination occur. Moreover, GPS cycles are typically run at temperatures greater than 1850 degrees C., thus favoring grain growth of large beta nuclei. Accordingly, GPS ceramics typically possess high toughness. However, because a GPS ceramic typically requires at least about 10 weight percent ("w/o") of sintering aids, its high temperature properties are usually mediocre.
In contrast, when a silicon nitride green body containing less than 5 w/o sintering aid is HIPped at about 1825 degrees C., it densifies to greater than 90% of theoretical density in as little as 15 minutes. Although the lower level of sintering aids in such hipped ceramics leads to superior high temperature properties, little grain growth occurs and microstructural development takes place only in the latter stages of densification. Since there is little opportunity for the formation of large beta nuclei, HIPped silicon nitrides do not possess the high aspect ratio grains required for high toughness.
Previous attempts to grow high aspect ratio grains in hipped silicon nitride included seeding the silicon nitride powder with beta grains and raising the HIPping temperature to at least 2000 degrees C. However, seeding was found to lower the average fracture toughness while the high temperature hipping modification increased the rate of silicon nitride sublimation, thus yielding ceramics having unacceptable levels of porosity.
Accordingly, it is the object of the present invention to provide a silicon nitride ceramic having low levels of sintering aids and a high toughness.